The present invention relates generally to laser, and more particularly to a method and apparatus for controlling the wavelength of a semiconductor laser (also referred to as a laser diode, or simply a laser).
A laser is an essential element in an optical communications system. The laser provides an optical carrier signal (i.e., the light) upon which data can be modulated and transmitted. Typically, the modulated optical signal is transmitted through a fiber optic cable to an optical receiver that demodulates the signal to obtain the data. Conventionally, only one optical signal (or one channel) is transmitted per optical fiber.
While demand for communications service increases, the capacity of optical communications systems is limited by the conventional one channel per optical fiber design. Although capacity can be increased by laying more fiber optic cables, the cost of such an expansion is prohibitive. The installation of optical fiber to interconnect various communications nodes represents a substantial investment of time and resources.
Wavelength division multiplexing (WDM) is a technology that increases the capacity of an existing fiber optic cable. A WDM system employs plural optical signal channels, with each channel assigned a particular wavelength. Each channel in a WDM system can be modulated at approximately the same data rate as a conventional single wavelength channel. Thus, the capacity of the fiber optic cable is increased by the number of channels utilized. For example, a four-channel WDM system has approximately four times the capacity of a single channel system. For the WDM system, the signal channels are (individually) generated, multiplexed together, and transmitted over the optical fiber. At the receiver, the WDM optical signal is demultiplexed and each channel is provided to an optical receiver that demodulates the channel to obtain the data.
Dense wavelength division multiplexing (DWDM) is a further extension of WDM and delivers even greater capacity. A DWDM system utilizes even more optical signal channels that are spaced more closely (i.e., the wavelengths are closer together) than a WDM system. Again, since each channel has a capacity that is approximately equal to that of the conventional single wavelength channel, the capacity of the DWDM system is increased by the ability to more closely space the channels.
The ability to multiplex a large number of wavelengths relatively close together is determined, to a large extent, by the ability to accurately control the wavelength of each transmitting source. As an optical signal in the DWDM optical signal drifts (i.e., in wavelength), it generates "crosstalk" and interferes with adjacent channels.
Accurate control of a laser wavelength is also important for other practical considerations. Conventionally, a DWDM transmitter is designed with a small range of wavelengths. The range can be expressed in terms of "wavelengths on ITU (International Telecommunications Union) grids", with each wavelength on the ITU grids corresponding to a difference of 0.8 nm (or 100 GHz) at a nominal wavelength of 1550 nm. The small range of the conventional DWDM transmitter is dictated by fundamental limits of the laser (e.g., a DFB laser) used within the transmitter and typically covers only two to three wavelengths on the ITU grids (i.e., approximately 200 to 300 GHz). To increase system reliability, many DWDM systems provide redundant (i.e., spare or backup) units that, collectively, covers the full span of the ITU operating wavelengths. Using conventional DWDM transmitter design, many spare units would be required because of the limited wavelength range of each unit.
Typically, the wavelength of a laser can be controlled by adjusting the temperature and the drive current of the laser. The drive current can be composed of several components, including a coupler current, a reflector current, and a phase current. Generally, the wavelength versus temperature and the wavelength versus drive current characteristics are not linear functions. In fact, these characteristics are often provided by the laser manufacturer by a special wavelength tuning algorithm or a special table, or both.
Various methods to control the wavelength of a laser have been developed in the art. For example, U.S. Pat. No. 5,706,301 to Lagerstrom discusses a laser wavelength control system that adjusts the laser wavelength by controlling the temperature of the laser. The temperature is controlled based on a difference in a transmitted optical signal (i.e., from the laser through a reflector) and a reflected optical signal (i.e., from the reflector). U.S. Pat. No. 5,553,087 to Telle discusses a frequency stabilized laser diode wherein the wavelength is set by adjusting the temperature of the laser diode until a certain condition is met and then adjusting the drive current of the laser diode. These conventional techniques generally adjust the laser wavelength by controlling temperature. Furthermore, these patents fail to account for the complex characteristics (i.e., the wavelength versus drive current) of the laser.